Rotary-wing aircraft employ a variety of means to interconnect a rotating rotor mast to a plurality of rotor blades, one of which is a yoke. Aerospace manufacturers face a multitude of conflicting design constraints when constructing new rotor yoke designs. For example, during flight, rotor yokes must all withstand powerful and repetitive stresses, such as torsional and centrifugal forces. In addition, weight remains a crucial constraint for all rotary-wing aircraft designs due to functional and operation cost concerns. Furthermore, aerospace manufacturers must meet these conflicting design constraints while simultaneously minimizing their costs. Carbon or glass fiber-reinforced composite materials (composite materials) present an attractive alternative for use by aerospace manufacturers to meet these conflicting design constraints.
Composites offer aerospace manufacturers an attractive alternative to metals or other materials due to their relative low cost, lightweight, reduced maintenance requirements, and high strength to weight ratios. The composite material used by aerospace manufacturers may comprise uniformly parallel, continuous fibers embedded in a bonding matrix material that form one or more layers (plies). The fibers provide the composite material with its strength and stiffness, which varies depending upon the fiber material chosen, such as carbon, glass, aramid, and polyethylene. Additionally, the stiffness and strength of the composite ply varies based upon the fibers' direction of loading within the design. The matrix, for example epoxy, phenolic, bismaleimide, and cyanate resins, binds together the fibers that give the composite material its shear strength. Thus, the fiber material, matrix, length, thickness, and cross section profiles of composite rotor yokes can be varied to produce very specific mechanical strength and properties.
Fiber steering is a manufacturing method aerospace manufacturers employ to create rotor yokes that uses unidirectional tape fiber composites. The tape is fed into a tow placement machine that steers the fibers along a curvilinear path through computer control according to a preprogrammed rotor yoke design. Thus, the fiber orientation angle may vary continuously throughout the structure within a given ply. This provides aerospace manufacturers with greater flexibility in achieving desired structural responses by the rotor yoke to the applied stresses experienced during flight.
Rotor yokes manufactured through fiber steering perform exceptionally with respect to the strength to weight ratio, lightweight, and reduced maintenance requirement design constraints. However, the manufacturing costs of such rotor yokes remain relatively high due to the inherent costs of the fiber steering manufacturing process. First, fiber steering manufacturing of rotor yokes requires the purchase of a tow placement machine, which can exceed several million dollars. Furthermore, as the tow placement machine must steer the tape for each path of the design, fiber steering manufacturing is a time intensive process. Additionally, fiber steering manufacturing is a labor intensive process as human operators must interact and monitor during the entire steering process. Also, as one hanging fiber can ruin a completed rotor yoke, the potential for human error adds considerable material waste to fiber steering manufacturing. Therefore, there is a need to develop a lower cost manufacturing method that is capable of maintaining exceptional strength, weight, and reduced maintenance characteristics.